Unfortunately some experimentation shows that creating a 2-in 4-out plugin doesn't work as a sidechain send. The plugin can be configured as both an effect and an instrument, meaning it can be placed inside an FX bin, as well as in the synth rack. When used in the synth rack, it's outputs become available as inputs to other tracks and busses.
However, when it is placed inside an FX bin (where we need it), it's outputs are no longer available as inputs to other tracks and busses.